Yes you can add. Create a new MSI based on the previous one with all the added registries and files and whatever changes you want.
Take Admin Install of both MSI. Then use the Patch creation tool to create MSP. It will first ask you for previous package, then new package and then will create MSP for you.
Read MSI.chm before creating it first time to get a better idea.

An application that has been installed using the Microsoft Windows Installer can be upgraded by reinstalling an updated installation package (.msi file), or by applying a Windows Installer patch (an .msp file) to the application.